All Bech32 addresses created by my Bitcoin Core installation are lower case.
I think OP is referring to the QR code displayed when clicking on "Create new receiving address" in the GUI.
If you scan the QR code, you will get an address with uppercase letters and "bitcoin:" prefix.
bitcoin:BC1Q..........
Doesn't make much sense to keep the address in uppercase as Bech32 and Base58 are designed to prevent human error in the first place. Maybe I will open a pull request for this.